1. Field of the Invention
The present invention relates to a method of nitriding steel for nitrogen case-hardening of steel which comprises subjecting a steel work to a special pretreatment that is conductive to a deep and uniform nitride layer or case.
2. Brief Description of the Prior Art
For the purpose of improving the wear resistance, corrosion resistance and mechanical properties such as fatigue strength etc. of steel, it is common practice to form a nitride layer or case on the surface of steel. Typical of this technique is the nitriding (gas nitriding, gas soft nitriding) process employing ammonia gas alone or a mixed gas composed of ammonia and a carbon source-containing gas (RX gas). Methods of this kind have problems with process stability in that when an alloy steel work or a steel work with an intricate configuration is treated, the resulting nitride case tends to be uneven.
While steel works are generally nitrided at temperatures not below 500.degree. C., the adsorption and diffusion of nitrogen on and into the surface layer of steel requires not only the absence of organic and inorganic stains but also the absence of an oxide film. Furthermore, the steel surface itself must be high in activity, too. Actually, however, it is impossible to prevent formation of an oxide film or obtain complete activation of the steel surface in such nitriding processes. Taking an austenitic stainless steel work as an example, it is generally cleaned with hydrofluoric acid-nitric acid to remove the passivation film from the surface prior to charge into the nitriding furnace but it is difficult to completely remove the passivation film and impossible to completely activate the surface layer of the steel. Therefore, it is near to impossibility to form a satisfactory nitride case. Moreover, the removal of organic and inorganic stains prior to nitriding is generally carried out by alkali degreasing or organic cleaning with, for example, trichloroethylene but the recent antipollution regulations (control against destruction of the ozonosphere) frustrate the practice of organic cleaning which is the most effective cleaning method so far available and this factor is also a major obstacle to the formation of a satisfactory nitride case.
Under the circumstances, the inventors of the present invention previously found that when a steel work prior to nitriding is first fluorided in heated condition under a fluorine-containing gas blanket such as NF.sub.3 and, then, nitrided, both the cleaning (removal of organic and inorganic stains and removal of the oxide film) and activation of the steel surface can be accomplished to give a satisfactory nitrogen case and a patent on the technology is pending (Japanese Patent Application No. 1-177660 and U.S. Ser. No. 479,013 filed on Feb. 12, 1990, now U.S. Pat. No. 5,013,371). In this method, the steel work is first heated and contacted with a gas, such as NF.sub.3, in a furnace for pretreatment. As a result, the organic and inorganic stain components adhering to the steel surface are destroyed by the activated fluorine atoms to leave a clean steel surface and, at the same time, the passivation film, inclusive of the oxide film, on the steel surface is converted to a fluoride film to cover and protect the steel surface. The steel work is then nitrided. In this nitriding process, the above fluoride film is destroyed and removed by introducing a mixed gas composed of a nitrogen source-containing nitriding gas (e.g. NH.sub.3 gas) and H.sub.2 gas into the furnace under heating. More specifically, the destruction and removal of said fluoride film leaves a clean and activated steel surface and the N atoms in the nitriding gas rapidly penerate and diffuse into this cleaned, activated steel to form a uniform and deep nitride case. However, despite the above-mentioned desirable performance characteristic of NF.sub.3 gas, it has the disadvantage of high cost. Moreover, a fairly high temperature (280.degree.-500.degree. C.) is required for adequate fluoriding and this means a significant thermal energy consumption, thus adding to the cost of treatment.